1) Field of the Invention
The present invention relates generally to agricultural implements, and, more specifically, to control of hydraulic depth control cylinders for such implements.
2) Related Art
Most available multi-frame implements such as field cultivators and chisel plows use a hydraulic cylinder system to provide depth control during field-working operations and adequate ground clearance during transport. Some depth control systems such as shown in U.S. Pat. No. 3,663,032 use rephasing cylinders connected in series to raise and lower the frames in unison. Other systems rely on mechanical depth stops to control operating height, but do not allow the operator to change that height from the cab or to easily vary depth for different transport conditions, ground contours and soil conditions. In addition, both the series cylinder and the mechanical depth stop systems require mechanical adjustment outside the cab to adjust the height of wing frames relative to the main or center frame. Adequate level control for fore-and-aft frame leveling or tilt and for leveling from side to side have presented problems.
In copending and commonly assigned U.S. application Ser. No. 08/972,925 filed Nov. 18, 1997 and entitled Electrohydraulic Control of Implement Lift Cylinders, we describe an improved lift system including a plurality of individual electrohydraulically controlled cylinders spaced on a tool frame and connected to frame suspension structure such as lift wheel assemblies. An implement control unit (ICU) in the towing vehicle cab is connected to feedback potentiometers or similar transducers which provide signals corresponding to cylinder stroke length or suspension position. The feedback signals are compared at the ICU to a control signal obtained by rotation of a lever or knob on an operator control unit in the cab. Depending on the cylinder position relative to the operator selected position, the ICU provides an output signal to the electrohydraulic valve to achieve a proportional amount of flow for the desired cylinder position. The ICU also compares feedback signals from the transducers and adjusts the output voltage to each of the valves so that the frame can be raised and lowered evenly. In conditions where independent adjustment of different frame portions is desired, the operator control unit can be adjusted to provide the necessary offset signal for each section. The lift system provides uniform depth control and level lift, even with dissimilar cylinders with different capacities and stroke lengths. Level lift and depth control can be achieved, relative to both the fore-and-aft and transverse directions.
One problem with an implement utilizing electronic controls is the inability to continue operations or to lift the implement for transport without a time consuming repair or an extensive replumbing of hydraulic hoses if there is a failure in the electronics, wiring harness or electrohydraulic valves. It is desirable to have a simple structure and straightforward, efficient method for continued operation of the implement in the event of such a failure.